Abstract

The fabrication and characteristics of close-packed lens arrays with a feature size close to the optical diffraction limit are presented in this study. By controlling the size of the submicron nickel rods and the time for reactive dry etching, the hemispherical lens array with a submicron period is made directly on a borosilicate glass. Finite-difference time-domain calculations and optical near-field measurements show that such a lens array can generate a subwavelength optical spot array near the glass surface. Moreover, the spot array periodically appears in the propagation direction. Using this novel optical property, we propose a photolithographic method for the mass-production of multilayer hexagonal structures with a period of 500nm.

Figures (7)

(a.) Anisotropic RIE method for making a microlens array on a glass substrate. The lens array is formed by longitudinally etching through a pre-shaped polymer mask. (b) Isotropic RIE method for making a submicron lens array. Submicron Ni-rod array is used as a hard mask. The submicron lens array is formed due to isotropic etching in the substrate.

(a). The SEM image of the Ni mask. The mask is composed of a hexagonal nickel rod array with 500nm-period and 150nm-thickness. (b). The SEM image of 30 minutes RIE in a borosilicate glass. A tapered rod array was formed. (c). The SEM image of 60 minutes RIE in a borosilicate glass. A hemispherical lens array was formed. (d). The enlarged image of Fig. 2(c), viewed at a tilt angle of 45°. (e). The SEM image of 90 minutes RIE in a borosilicate glass. Pyramid shape with rough surface was formed. (f). The SEM image of 60 minutes RIE in a fused silica. The etching is highly anisotropic, resulting in a high-aspect-ratio rod array.

(a). The layout for the FDTD calculations. The array was composed of hemispherical glass lenses with a 250nm radius and a 500nm period. The refractive index was 1.5 for glass and the incident light was 442nm. (b). The calculated optical field distribution at the XZ plane. The lens array shows focusing spots near the lens surface. The spot has a diameter ~250nm close to the diffraction-limit. Note that the spot array appears periodically in the z-direction. (c). The optical field distributions at XY plane at different z positions: 0.5μm, 1μm and 1.5μm from the lens surface. The focusing spots show alternative hexagonal patterns in the propagation direction.

The XZ plots of the optical distributions for different periods of lens arrays. (a) 300nm-period array. The curvature of lens is too large that most light is confined in the lens. (b) 700nm-period array. The focusing behaviors are similar to the 500nm-period array, but with a longer focusing period in z- direction. (c) 900nm-period array. The curvature of lens is too small to form a tight spot.

(a). A simple diagram for the setup of a collection-mode NSOM. A 473nm laser was incident into the glass substrate. The optical intensity was collected by a tapered fiber probe. (b) The topographic image of the submicron lens array. (c) The corresponding optical near-field distribution. (d) The measured optical intensity distribution in the XZ plane.

(a). The setup for photolithography using a submicron lens array as a photomask. (b) The SEM images for developed pattern in the photoresist and cross-section of the hole pattern viewed at 45° tile angle.